A step-up DC voltage converter is known whose basic circuit comprises a storage choke, the one terminal of which is connected to the input of the converter and the other terminal of which is connectable via a controllable switch to ground and is connected to the anode of a diode, the cathode of which is connected to the output of the converter. The circuit comprises in addition an output capacitor connected between the output of the converter and ground. One such basic circuit of a step-up DC voltage converter is described e.g. in the German textbook "Halbleiterschaltungstechnik" by U. Tietze and Ch. Schenk, 11th Edition, published by Springer-Verlag, Berlin, Heidelberg, 1999 on pages 985-986.
Such a converter requires in addition a clock, the clock signal of which dictates the switching cycle of the converter and a regulator circuit which regulates the output voltage of the converter to a predetermined setpoint value. Furthermore, a control circuit, e.g. a logic circuit needs to be provided which controls the switch with the aid of the clock signals of the clock and with the output signal of the regulator circuit.
In accordance with one known method, described e.g.in U.S. Pat. No. 5,481,178, the step-up DC voltage converter as described above may be operated as follows:
On commencement of each switching cycle the switch is set ON so that energy is stored in the storage choke, the current flowing through the storage choke increasing linearly. In this arrangement the regulator circuit monitors this current and sets the switch OFF when a specific current threshold which depends on the existing load situation is attained, resulting in the energy stored in the storage choke being output to the output of the converter in a second phase of the switching cycle. The choke current dropping linearly in this phase from its peak value attained at the end of the first phase flows through the diode into the output capacitor and a consumer connected to the output of the converter. In this arrangement the storage choke and its input voltage acts like a series connection of two voltage sources, this being the reason why the output voltage increases by the storage choke voltage relative to the input voltage.
The current threshold value is dictated by an error amplifier outputting a value proportional to the difference between a predetermined reference voltage and a voltage proportional to the actual output voltage of the converter, this value being compared in a comparator to a voltage value proportional to the current flowing through the storage choke. The output signal of the comparator controls the ON time of the switch.
The regulator circuit results, in all, in a pulse duration modulation of the ON time of the switch to achieve the desired setpoint output voltage of the converter. On commencement of the next switching cycle as dictated by the clock signals of the clock the switch is then returned ON and the method recommenced.
In this known method the step-up DC voltage converter is additionally operated as a function of the output load current in three different modes:
When the load current is high, the converter is operated in a first mode, the mode of continual storage choke current flow in which a permanent current flows through the storage choke (see FIG. 1a). In this arrangement the switch is cycled ON/OFF, whereby in the second phase of the switching cycle in which the switch is set OFF the storage choke current never drops to zero. The peak-to-peak output voltage ripple in this mode is very small.
With a reduction in the load current, the average current flowing through the storage choke must automatically also become less. At some point in time the average storage choke current then becomes so small that in the second phase of the switching cycle and prior to the end thereof it drops to zero (see also FIG. 1b). This is the second mode, the mode of interrupted storage choke current flow. The diode in this mode prevents a reverse current once the storage choke current has dropped to zero.
If the diode is replaced by a second switch which may be of advantage in certain applications, e.g. to reduce the energy losses occurring due to the diode, it needs to be signalled OFF following the drop in the storage choke current to zero, meaning it would need to be signalled ON again in the next switching cycle following OFF of the first switch.
In the two modes described a relatively high efficiency of the known step-up DC voltage converter is achievable. Since the converter is operated in these two modes at a defined known switching frequency, the frequency occurring in the voltage ripple is likewise known, thus making it a relatively easy task to filter out the noise appearing at the output. The ripple of the output voltage comprises in these modes no low-frequency components which is a salient requirement for the use of such converters in telecommunication devices, e.g. in mobile telephones.
So that the regulation function is still available in the mode of interrupted storage choke current flow the controllable switch needs to be ON at least for a certain minimum time duration, i.e. as long as the comparators of the regulator circuit and the logic circuit of the control circuit have sufficient time to settle at specific levels. When the load current requirement in the interrupted storage choke current current flow mode is very low it may be that the ON time of the switch required as a minimum for settling is too high. Then, in the first switching phase of the switching cycle, during which the controllable switch is set ON, more energy would be stored in the choke than is needed at the time for achieving the load current. In this case the converter would no longer be able to regulate the output voltage, i.e. the output voltage would violate its defined setpoint value.
To get round this problem and to achieve good efficiency of the converter even when the load current is very small or non-existent, the converter as known from U.S. Pat. No. 5,481,178 is operated in a third mode, a so-called skip mode. To adapt the energy stored in the storage choke to a very low or non-existent load current requirement in the skip mode individual switching cycles in which no energy is stored in the choke are skipped and thus also no energy can be passed on to the converter output (see FIG. 1c). The controllable switch is thus set ON e.g. only for every 2, 3 or only every 10 switching cycles. The number of switching cycles skipped depends on the level of the load current needed at the time, whereby a comparator may be provided which monitors the output voltage and signals the converter ON as soon as a critical comparison value is no longer attained and OFF as soon as this comparison value is exceeded.
However, the skip mode has numerous disadvantages especially in the case of step-up DC voltage converters when intended for use in telecommunication devices. Thus, in the skip mode the step-up DC voltage converter is activated with an irregular frequency and the ripple of the output voltage comprises low-frequency components which makes noise filtering difficult. As compared to the two other modes the ripple of the output voltage in the skip mode is also stronger, the reason for this being that the output voltage is regulated only by a simple bang-bang circuit. In conclusion, an additional circuit arrangement is needed to shuttle the step-up DC voltage converter between the interrupted storage choke current current mode and the skip mode.